1. Field of the Invention
The present invention relates generally to Doppler radar, and more particularly to an FM pulse Doppler radar apparatus, such as a radar apparatus mounted on a vehicle, for determining the distance to an object.
2. Description of the Related Art
As for this type of radar apparatus, an FM pulse Doppler radar illustrated in FIG. 10 is known in the art. Referring to FIG. 10, the FM pulse Doppler radar apparatus includes a modulation-voltage generator 1, a voltage-controlled oscillator (triangular wave generator) 2 for generating electromagnetic waves having transmission frequency ftx of, for example, 76 to 77 GHz, a transmit-receive switch 3 for switching the output feed of the electromagnetic waves generated by the voltage-controlled oscillator 2 between a transmitting amplifier 4 and a receiving mixer 9, the transmitting amplifier 4 for amplifying the power of the electromagnetic waves fed by the transmit-receive switch 3, and a transmitting antenna 5 for radiating the electromagnetic waves amplified by the transmitting amplifier 4.
Further in FIG. 10, an object 6 is a target to be detected by the radar. The apparatus also includes a receiving antenna 7 for receiving the electromagnetic waves radiated to and reflected from the object 6, and a receiving amplifier 8 for amplifying the received electromagnetic waves.
The apparatus further includes the mixer 9 for outputting beat signals corresponding to the distance and relative velocity of the object 6 by mixing transmission electromagnetic waves switched by the transmit-receive switch 3 and electromagnetic waves reflected back from the object 6, a low-pass filter 10 of which the cut-off frequency is the inverse of the transmission pulse time width, an AGC amplifier 11 capable of controlling the gain according to the received power of the reflected waves, an A/D converter 12 for converting the beat signals into digital signals, and a distance calculator 13 for calculating the distance and relative velocity of the object 6 based on the A/D values.
An electromagnetic wave transmitting operation of a typical known radar apparatus having the above-described structure will now be described.
First, the voltage-controlled oscillator 2 outputs electromagnetic waves (triangular wave signals) modulated, for example, as in FIG. 2, corresponding to the voltage signals from the modulation-voltage generator 1. The electromagnetic waves output from the voltage-controlled oscillator 2 are fed to the transmitting amplifier 4 by the transmit-receive switch 3 and are amplified therein. The electromagnetic waves amplified by the transmitting amplifier 4 are radiated from the transmitting antenna 5.
Next, an electromagnetic wave receiving-operation will be described. The transmit-receive switch 3 is switched to the receiving side to connect the voltage-controlled oscillator 2 and the mixer 9 when a pulse time width Tg, for example, of 33.3 ns (=1/30 MHz, equivalent to a distance of 5 m) has elapsed from the time electromagnetic wave transmission was initiated. The electromagnetic waves sent from the transmitting antenna 5 form pulse waves, each pulse having a duration of 33.3 ns. The pulse waves are reflected ago by the object 6 at a distance R, and input to the receiving antenna 7 after a delay time xcex94t depending on the distance R relative to the transmitted electromagnetic waves.
When the object 6 has a relative velocity, the frequency of the received electromagnetic waves is Doppler-shifted relative to the frequency of the transmission electromagnetic waves, and they are input to the receiving antenna 7. The electromagnetic waves input to the receiving antenna 7 are amplified by the receiving amplifier 8 and mixed with the transmission electromagnetic waves from the voltage-controlled oscillator 2 by the mixer 9 to output beat signals shown in FIG. 3. The beat signals thus acquired pass through the low-pass filter 10, of which the cut-off frequency is, for example, 30 MHz, are amplified by the AGC amplifier 11, are input in the A/D converter 12, and are converted to digital signals.
Next, a method for calculating the distance and relative velocity of the object 6 by the distance calculator 13 based on the output data from the A/D converter 12 is explained.
For the purpose of understanding, it is assumed that the voltage-controlled oscillator 2 does not perform FM modulation and the transmission frequency ftx is fixed to 76.5 GHz. In order to obtain a velocity resolution of 1 km/h, a Doppler frequency resolution xcex94f is:                               Δ          ⁢                      xe2x80x83                    ⁢          f                =                                            2              ⁢              Δ              ⁢                              xe2x80x83                            ⁢              v                        λ                    =                                                    2                xc3x97                0.2777                ⁢                                  xe2x80x83                                ⁢                                  m                  /                  s                                                            0.003921                ⁢                                  xe2x80x83                                ⁢                m                                      =                                          141.64                ⁢                                  xe2x80x83                                ⁢                                  (                  Hz                  )                                            =                                                1                                      7.05977                    ⁢                                          xe2x80x83                                        ⁢                                          (                      ms                      )                                                                      =                                  1                  Tm                                                                                        (        1        )            
and a calculation time Tm of approximately 7.06 ms is required. If, for example, the maximum detection range is set to 150 m, then the transmission wave output cycle is 33.3 nsxc3x97(150/5)=1 xcexcs. In order to obtain the velocity resolution of 1 km/h, the above-described device acquires, at each range gate, beat signals for the transmitted waves output 7060 times, as in FIG. 4, and performs, for each of the range gates, a fast-Fourier-transform of all the data. Then the beat frequency at a particular range gate is output as shown in FIG. 5.
The distance Rg and the relative velocity V may be calculated using equations (2) and (3) below:                     Rg        =                              tg            xc3x97            n            xc3x97            C                    2                                    (        2        )                                V        =                              fb1            xc3x97            C                                2            xc3x97            f0                                              (        3        )            
where tg is a range gate time width (pulse time width), n is a range gate number, c is the speed of light, fb1 is a Doppler frequency (=beat frequency), and f0 is the transmission frequency (76.5 GHz).
Now, consider that the above-described transmission electromagnetic waves are modulated as in FIG. 2. Suppose that during the above-described calculation time Tm of approximately 7.06 ms, the transmission frequency is increasing steadily from 76.425 to 76.575 GHz, the bandwidth B being 150 MHz. The time t required for the electromagnetic waves to be transmitted from the transmitting antenna 5, reflected from the object 6 and input to the receiving antenna 7 may be found by the following equation (4):                     t        =                              distance            xc3x97            2                    C                                    (        4        )            
As the transmission frequency is increasing steadily during the time t, the beat frequency fbu is found by summing the Doppler frequency fb1 due to the relative velocity and fb2 which represents the difference between transmission frequency and the received frequency corresponding to the distance, as follows:
fbu=fb2+fb1xe2x80x83xe2x80x83(5)
Likewise, suppose that the transmission frequency is steadily decreasing from 76.575 GHz to 76.425 GHz, the bandwidth B being 150 MHz, during the next calculation time Tm of approximately 7.06 ms. As the transmission frequency is decreasing during time t required for the electromagnetic waves to be output from the transmitting antenna 5, reflected from the object 6 and input to the receiving antenna 7, the beat frequency fb1 may be obtained by summing a Doppler frequency fb1xe2x80x2 due to the relative velocity and fb2xe2x80x2 which represents the difference between the transmission frequency and the received frequency corresponding to the distance. The interval of increasing/decreasing the frequency, the distance, and the relative velocity between the radar and the object may be the same as in the foregoing case of increasing the frequency. Because the bandwidth remains constant and the increase/decrease rate is equal, fb1=fb1xe2x80x2 and fb2xe2x80x2=xe2x88x92fb2. The beat frequency fbd may be obtained by equation (6) below:
fbd=fb2xe2x80x2+fb1xe2x80x2=xe2x88x92fb2+fb1xe2x80x83xe2x80x83(6)
As the beat frequency fbu can be obtained by increasing the transmission frequency and the beat frequency fbd can be obtained by decreasing the transmission frequency as described above, the Doppler frequency fb1 due to the relative velocity, and fb2 which represents the difference between the transmission frequency and received frequency corresponding to the distance may be found by equation (7) below:                               fb1          =                                    fbu              +              fbd                        2                          ,                  fb2          =                                    fbu              -              fbd                        2                                              (        7        )            
As fb2 represents a frequency increase or decrease during the time t in equation (4), the following equation (8) can be derived:                               fb2          B                =                  t          Tm                                    (        8        )            
Using equations (4) and (8), a distance Rb can be obtained from fb2 as in the following equation (9):                     Rb        =                                            Tm              xc3x97              C                                      2              xc3x97              B                                xc3x97          fb2                                    (        9        )            
It is apparent from equation (9) that the distance Rb is proportional to fb2. A distance resolution is xcex94R and a frequency resolution of fb2 is xcex94f(=1/(Tm/2) are obtained as follows:                               Δ          ⁢                      xe2x80x83                    ⁢          R                =                                                            Tm                xc3x97                C                                            4                xc3x97                B                                      ⁢            Δ            ⁢                          xe2x80x83                        ⁢            f                    =                      C                          2              xc3x97              B                                                          (        10        )            
By increasing B to 300 MHz, a resolution xcex94R of 0.5 m, a higher resolution than the distance Rg, is achieved.
Even when the beat frequency due to noise arising from various sources is detected at a certain range gate, it can be identified as noise and eliminated if the difference between the distance Rg obtained by equation (2) and the distance Rb obtained by equation (9) is a range gate width of 5 m or more.
In theory, the distances obtained by equations (2) and (9) should coincide within the scope of a range gate width. However, in reality, errors in the transmission frequency bandwidth B occur due to variation in component devices or temperature changes, which cause errors in the distance Rb obtained by equation (9).
For example, when variation in component devices causes the transmission frequency bandwidth B to be 1.1 times the prescribed width, the distance obtained by equation (9) is:                     Rg        =                                                            Tm                xc3x97                C                                            2                xc3x97                                  (                                      1.1                    xc3x97                    B                                    )                                                      xc3x97            fb2                    =                                                    Tm                xc3x97                C                                                              2                  xc3x97                  B                                )                                      xc3x97            fb2            xc3x97                          1              1.1                                                          (        11        )            
which is 1/1.1 times the correct distance.
Likewise, when the transmission frequency bandwidth B is varied from 0.9 to 1.2 times the prescribed width due to temperature changes, the distance obtained by equation (9) also varies from 1/0.9 to 1/1.2 times the correct distance.
One way to avoid such difficulties is to minimize the difference between elements or temperature changes as much as possible. In order to do so, however, it is necessary to use expensive materials or to detect and adjust the error in the bandwidth B of individual apparatuses at the time of assembly.
Accordingly, it is an object of the present invention to provide an FM pulse Doppler radar apparatus which detects an error even when there is an error in a transmission frequency bandwidth B, and which effectively corrects the error based on that error. When the error is large, the apparatus determines that the apparatus is functioning abnormally.
It is another object of the present invention to provide an FM pulse Doppler radar apparatus that accurately identifies whether the source of error is noise, a distance error, or an abnormality of the apparatus.
Still another object of the present invention is to provide an FM pulse Doppler radar apparatus that accurately detects the distance error and corrects a distance calculation result based on the detected error.
Yet a further object of the present invention is to provide an FM pulse Doppler radar apparatus capable of calculating an accurate correction value.
According to the present invention, the foregoing objects are achieved through provision of an FM pulse Doppler radar apparatus which performs pulse modulation of modulating waves having repeatedly increasing and decreasing frequency, which transmits the thus modulated waves, receives, at each range gate having an interval equivalent to a pulse width, reflected waves reflected by at least one object, determines a distance corresponding to the range gate, and calculates a distance to the at least object and the relative velocity of the at least one object based on a difference between the frequencies of the transmission waves and the received waves. The FM pulse Doppler radar apparatus includes a velocity determining unit for determining the velocity of a vehicle on which the FM pulse Doppler radar apparatus is mounted, and a comparison-and-detection unit for comparing the obtained distance corresponding to the range gate and a distance calculated based on the difference between the frequencies of the transmission waves and the received waves, and for detecting, based on the velocity of the radar-mounted vehicle and the relative velocity between the vehicle and the at least one object, a distance error due to an error in transmission frequency bandwidth.
Preferably, the comparison-and-detection unit detects the distance error due to the error in the transmission frequency bandwidth by comparing the detected range gate number and a range gate number obtained by an inverse operation from the distance calculated from the difference between the frequencies of the transmission waves and the received waves.
The comparison-and-detection unit may temporally total distance errors due to each error in the transmission frequency bandwidth obtained from a plurality of the objects, and may find the average of the total.
The comparison-and-detection unit may detect the distance error due to the error in the transmission frequency bandwidth, and may correct the distance calculation result based on the distance error.
The comparison-and-detection unit may correct the distance calculation result by detecting the distance error due to the error in transmission frequency bandwidth and by correcting the error in the bandwidth of the transmission frequency based on the detected distance error.
The comparison-and-detection unit may detect the distance error due to the error in the transmission frequency bandwidth only when the object used for the error detection is stationary.
The comparison-and-detection unit may detect the distance error due to the error in the transmission frequency bandwidth only when the radar-mounted vehicle as well as the object used for the error detection are stationary.
The comparison-and-detection unit detects the distance error due to the error in the transmission frequency bandwidth only when the object used for the error detection is a travelling vehicle being followed by the radar-mounted vehicle.
The comparison-and-detection unit may temporally adds up distance data on each of the objects calculated from the difference between the frequencies of the transmission waves and the received waves at each range gate, and may detect the distance error due to the error in the transmission frequency bandwidth from the frequency distribution of the distance data.
When the distance error due to the error in the transmission frequency bandwidth may exceeds a predetermined value, the comparison-and-detection unit may determine that the apparatus is functioning abnormally.